1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, printers, facsimiles and the like, and particularly to an image forming apparatus which can effectively consume oppositely charged toner of a development unit to perform development by a single component development system.
2. Description of the Related Art
In a non-contact development system in which single component non-magnetic toner is carried on a development roller as a developer and the toner is supplied to an electrostatic latent image formed on a photoreceptor to develop the image, a developing bias voltage formed by superimposing a direct current voltage on a pulsed alternating voltage is applied to the development roller. This developing bias voltage made of a development voltage and a recovery voltage. The toner is subjected to a force from the development roller toward the photoreceptor by the development voltage, and the toner is drawn back from the photoreceptor toward the development roller by the recovery voltage. The toner to which these development voltage and recovery voltage are applied alternately adheres to the electrostatic latent image on the photoreceptor to perform development.
When the single component non-magnetic toner on the development roller passes between the development roller and a regulation blade or a supply roller contacting with the development roller, a part of the toner deteriorates in charge characteristic due to friction and becomes oppositely charged toner which is charged oppositely to a normally charged toner. The oppositely charged toner causes image defects of adhering to a non-image forming section to cause fog without contributing to development and interfering with flying of the normally charged toner to reduce an image density and cause image irregularities. Particularly when printing at a low coverage rate, image defects become outstanding.
In Japanese Unexamined Patent Publication No. 2001-75438, a consumed amount of toner per unit drive time of the development roller or per unit revolution of the development roller is determined, and if this consumed amount of toner is small, that is, when it is found to be low in a coverage rate, the toner carried on the development roller is compulsorily discharged toward the photoreceptor to be consumed, and thereby the occurrence of the fog of toner or image irregularities is prevented.
However, in a constitution of Japanese Unexamined Patent Publication No. 2001-75438, it is inefficient since a predominant large amount of the normally charged toner is simultaneously discharged with the oppositely charged toner when compulsorily consuming the toner and therefore the rate at which the oppositely charged toner actually wanted to be consumed is discharged is small. Further, since the normally charged toner having a high developing property is also discharged and disposed of, this method is low in cost-effective and disadvantageous for users.
In Japanese Unexamined Patent Publication No. 2004-29104, it is proposed to compulsorily consume the oppositely charged toner in a non-image region on an image-carrier based on a consumed amount of a developer. Specifically, in the case of normally developing, the photoreceptor is charged to a charging potential (−450 V) and a portion to be a black image is diselectrified to an exposure potential (−50 V) to form an electrostatic latent image, and on the other hand, the negatively charged toner on a development sleeve is flown toward the electrostatic latent image on the photoreceptor by applying a developing bias voltage (−350 V) to the development sleeve. When the so-called reverse development, in which the oppositely charged toner is consumed, is performed, the charging potential of the photoreceptor is changed to −800 V in a region where images are not formed to compulsorily fly the positively charged toner on the photoreceptor onto the photoreceptor by a Coulomb force.
However, an adhesion force in a direction of the development roller such as an image force and a Van der Waals force is exerted on the toner on the development roller in addition to a Coulomb force. As shown in FIG. 11A, the Coulomb force increases linearly as the toner charged amount become larger. The image force increases in a quadratic manner as the charged amount of the toner increases but the Van der Waals force is constant regardless of the charged amount of the toner. Therefore the adhesion force which combines the image force and the Van der Waals force also increases in a quadratic manner with respect to the charged amount of the toner. Therefore, the Coulomb force becomes smaller than adhesion force whether a charged amount is small or large, and the toner cannot fly. A range of the charged amount at which the toner can fly is limited. As is apparent from a distribution of the charged amount of the toner shown in FIG. 11B, the oppositely charged toner generally has a small absolute value of the charged amount and a small Coulomb force.
In the foregoing Japanese Unexamined Patent Publication No. 2004-29104, the Coulomb force is increased as far as possible by enhancement of an electric field and thereby the toner is flown, but the oppositely charged toner which actually overcomes the adhesion force and can arrive at the photoreceptor is limited to infrequent toner having a large absolute value of the charged amount. Accordingly, an effect of compulsorily consuming the toner is small.
It is an object of the present invention therefore to provide an image forming apparatus which can fly the toner toward a photoreceptor to compulsorily consume the toner even though the toner is oppositely charged toner having a small charged amount and thereby can prevent image defects such as fog and irregularities caused by the accumulation of the oppositely charged toner.